There's also the flipside of that: rather than "higher" English, native speakers are also more likely to use fairly "sloppy" English, especially with pronunciation. When not thinking about it and speaking quickly, native English speakers, or at least American English speakers, tend to elide or merge syllables, which second-language speakers often find challenging to understand.
I've more than once found that the reason someone didn't understand something I said in English was just because, speaking quickly, I was leaving out 20-30% of the syllables, if you were to really try to pick out distinct ones. Americans hearing me speak would fill these in subconsciously, but second-language speakers can be thrown off. For example, if you listen to how I say a phrase like "no, morning won't work, because of the meeting" (11 syllables) when I'm speaking very quickly, you can only really make out 7-8 distinct syllables, roughly "nah, morn' won' work 'cause the meet'n".
Yes this is true - I guess I am just expressing my personal experience where I forget I am speaking to non-native speakers and use language they are unfamiliar with.
I've more than once found that the reason someone didn't understand something I said in English was just because, speaking quickly, I was leaving out 20-30% of the syllables, if you were to really try to pick out distinct ones. Americans hearing me speak would fill these in subconsciously, but second-language speakers can be thrown off. For example, if you listen to how I say a phrase like "no, morning won't work, because of the meeting" (11 syllables) when I'm speaking very quickly, you can only really make out 7-8 distinct syllables, roughly "nah, morn' won' work 'cause the meet'n".